Conventionally, in order to control the ON/OFF of a load such as a lighting apparatus, a ventilation fan or the like, a load control device (an electronic switch) using a non-contact switch device such as a triac has been put to practical use instead of a two-wire switch in which contacts are mechanically closed or opened (see, e.g., Japanese Patent Application Publication No. 2008-97535). Such a load control device generally employs a two-wire connection to reduce a wire and is connected in series between a commercial power source (an AC power source) and a load. In the case of the load control device connected between the commercial power source and the load, how to acquire its own circuit power is an issue.
FIG. 10 shows the circuit configuration of a conventional two-wire load control device 50 that is connected in series between a commercial power source 2 and a load 3. This load control device 50 includes primary and auxiliary switching units 51 and 57 configured to control the ON and OFF of the load 3, a control unit 53 configured to control the conduction of the primary and auxiliary switching units 51 and 57, and a power supply circuit configured to supply a driving power to the control unit 53. The power supply circuit includes a rectifying unit 52, a first power supply unit 54 configured to stabilize supply of a power to the control unit 53, a second power supply unit 55 configured to supply a power to the first power supply unit 54 when the power supply to the load 3 is stopped, and a third power supply unit 56 configured to supply a power to the first power supply unit 54 when the power supply to the load 3 is performed. The auxiliary switching unit 57 includes, e.g., a thyristor 57a, which functions to allow a main circuit current to flow into the load 3 when the capacity of the load 3 is small and also only a current that is lower than the holding current required to maintain the conduction of the triac (main switch device) 51a of the primary switching unit 51 flows.
The second power supply unit 55 is a constant voltage circuit that includes, e.g., a resistor configured to limit a current, a Zener diode (constant voltage diode) 55a configured to clip a voltage and the like. A ripple current which has been full-wave rectified by the rectifying unit 52 is inputted into the second power supply unit 55. Further, only when the voltage value of the inputted ripple current is higher than the Zener voltage of the Zener diode 55a, a current flows therethrough. A part of the current flows into the first power supply unit 54 to be supplied as a power of the control unit 53 and is charged in a buffer capacitor 54a connected to the input terminal of the first power supply unit 54. When the voltage of the ripple current full-wave rectified by the rectifying unit 52 is lower than the Zener voltage, the buffer capacitor 54a becomes a power source, and thus supplies a power to the first power supply unit 54. Accordingly, the buffer capacitor 54a repeats charging and discharging.
In other words, even when the load 3 is in an OFF state, a current flows into the load 3 via the Zener diode 55a and the rectifying unit 52. In this case, the current flowing into the load 3 needs to have a small magnitude that prevents an erroneous operation of the load 3. Further, the current consumption of the control unit 53 is set to be kept small and the impedance of the second power supply unit 55 is set to be kept high. Further, the first power supply unit 54 functions as a voltage stabilization unit.
Meanwhile, when an operating handle SW 4 that activates the load 3 is manipulated, the control unit 53 outputs a control signal to make the switch device 56c of the third power supply unit 56 to be conductive, whereby the buffer capacitor 54a is charged. When the buffer capacitor 54a is charged, the current passes through the Zener diode 56a, the thyristor 57a of the auxiliary switching unit 57, and the triac 51a of the primary switching unit 51 in sequence. When the triac 51a becomes ON, the rectification voltage of the rectifying unit 52 becomes almost zero, and thus, the second power supply unit 55 becomes non-conductive and no current flows. The same is true of the third power supply unit 56.
While the second power supply unit 55 and the third power supply unit 56 are non-conductive, the first power supply unit 54 is supplied with a power from the buffer capacitor 54a, and thus, the input voltage of the first power supply unit 54, i.e., the terminal voltage of the buffer capacitor 54a, is gradually decreased. Meanwhile, when the current flowing into the triac 51a becomes 0, the triac 51a is caused to enter an open state (a non-conductive state) by self arc-extinguishing and a voltage is generated in the rectifying unit 52. When the voltage increases above the terminal voltage of the buffer capacitor 54a, the buffer capacitor 54a starts to be charged. Since the impedance of the second power supply unit 55 is set to be a value sufficiently higher than that of the third power supply unit 56, the second power supply unit 55 does not contribute to the operation of the load control device 50 while the load 3 is ON.
Once the primary switching unit 51 has become conductive (a closed state), it continuously flows the current. However, when the commercial current reaches a zero-cross point, the triac 51a is self arc-extinguished, and the primary switching unit 51 becomes non-conductive (an open state). When the primary switching unit 51 becomes non-conductive, an operation of acquiring the circuit power by the load control device 50, in which the current flows from the rectifying unit 52 through the third power supply unit 56 to the first power supply unit 54, is performed. That is, in every half period of AC, the operation of acquiring the circuit power by the load control device 50, and the conduction of the auxiliary switching unit 57 and the conduction of the primary switching unit 51 are repeated.
However, in order to reduce the power consumption, replacement to LED (Light-Emitting Diode) lamps has been conducted. Since an LED device emits a light by using DC, a power supply circuit configured to convert AC into DC is provided in the LED lamp. However, there are power supply circuits for some loads, such as inexpensive LED lamps, which have no countermeasures against noise (e.g., the parallel connection of a capacitor or the like between the terminals of a power supply circuit). When an LED lamp without noise countermeasures is connected as a load to the two-wire load control device 50, a current flows into the load because of the acquisition of the load control device 50's own circuit power even when the load needs to be in an OFF state. Therefore, there is a possibility of the erroneous operation of the load (e.g., flickering of the LED lamp). Further, in the conventional load control device 50, a voltage is stepped down by the Zener diode 55a, and thus, an energy corresponding to the voltage step-down is consumed by thermal conversion. Accordingly, the conventional load control device 50 does not contribute to the improvement of energy efficiency.